A Single Input Single Output (SISO) technique is used in 1st Generation (1G) to 3rd Generation (3G) cellular wireless communications. The SISO technique uses one antenna between transmitting and receiving ends. On the other hand, to increase a data rate by utilizing limited frequency resources and bandwidths, in a post-3.5G wireless communication technique, a Multiple Input Multiple Output (MIMO) technique for improving a spectral efficiency (bits/Hz) by using multiple antennas is commercialized. In particular, as one of representative core technologies of 4G wireless communications, the MIMO technique is provided under the premise that multiple antennas must be supported by configuring multiple radio frequency (RF) chains constituting a mobile station (MS) and a base station (BS).
In a system using the MIMO technique, an entire data stream to be transmitted is divided into several data streams. The divided data streams are individually modulated and are simultaneously transmitted with the same frequency through respective transmit (Tx) antennas and RF chains. In this case, reflected multi-path signals are utilized to improve transmission characteristics, which is the MIMO technique. Respective receive (Rx) antennas and RF chains receive multiple Tx data streams through the multi-path signals. The received data streams are restored by using a MIMO algorithm calculated based on Tx/Rx channel state information.
A 2×2 down link (DL) MIMO system has a structure as follows. When signals are received as multi-path signals from a transmitter through two Rx antennas, a receiver delivers the signals to a baseband modem through independent RF chains and an analog to digital converter (ADC). Referring to FIG. 1, when a transmitter transmits a data stream 1 through a 1st Tx RF chain 101-1 and a 1st Tx antenna 103-1 and transmits a data stream 2 through a 2nd Tx RF chain 101-2 and a 2nd Tx antenna 103-2, a 1st Rx antenna 105-1 of a receiver delivers a data stream 1′ and a data stream 2′ to a baseband modem (not shown) through a 1st Rx RF chain 107-1 and a 1st ADC (not shown). Further, a 2nd Rx antenna 105-2 of the receiver delivers a data stream 1″ and a data stream 2″ to the baseband modem (not shown) through a 2nd Rx RF chain 107-2 and a 2nd ADC (not shown). In this case, the baseband modem (not shown) separates input signals into streams to restore the original data streams 1 and 2.
It is important herein that two independent Rx RF chains and antennas are required to implement a 2×2 MIMO system. Each Rx RF chain includes a low noise amplifier (LNA), an in-phase/quadrature (I/Q) demodulator, a mixer, an analog filter, an automatic gain controller (AGC) amplifier, and so forth. The data streams 1 and 2 can be separated and restored when signals, which are not electrically mixed, are delivered to the baseband modem through the two independent Rx RF chains and antennas. The signals cannot be delivered by sharing one antenna and one RF chain. Accordingly, an additional RF chain causes more power consumption. Further, a die size of a radio frequency integrated circuit (RFIC) increases. In general, the analog filter and the ADC occupy 50% of the die size of the RFIC. In addition, the additional RF chain consumes power of at least 100 mW. The increase in the die size of the RFIC results in a high chip cost, which may have a significant effect on the costs of constitutional parts of MSs and equipments when a 4×4 MIMO system is implemented in the future.
Accordingly, there is a need for a method capable of achieving a low chip cost and power saving by simplifying a structure of an RF chain constituting the MIMO system.